DC-to-AC power converters are often used in uninterruptible power supplies. This is because one common method of storing back-up power is through the use of rechargeable batteries, which store and disburse power as DC energy. Correspondingly, when the main power source fails and the uninterruptible power supply provides back-up AC power from the battery supply, the stored DC energy needs to be converted into AC power.
Furthermore DC energy is often stored in the uninterruptible supply at voltage levels below the typical peak-to-peak line voltage levels of the main power supply. Correspondingly, when stored DC energy is converted into AC power the voltage level needs to be boosted or stepped up. One or more transformers are commonly used for boosting or stepping up the voltage level. At least one technique allows for varying the winding ratio of the primary and secondary windings for adjusting the voltage levels and/or the current levels.
In many instances uninterruptible power supplies provide back-up power for motors, one example being sump pump motors. During the start up time of a single phase motor, the torque required to initiate shaft rotation of the motor must exceed the torque demanded by the motor load. This causes the current in the motor winding to increase, in many instances, by a factor of 6 or 7 times the motor's rated current. Generally, this current exists for between 15 and 20 sixty hertz cycles, or between 250 ms and 333 ms. For power supplied by the local utility, this typically has no effect on the power quality.
However for switch mode power supplies, consistent with those used in uninterruptible power supplies, a large current draw from a motor starting can result in immediate shut down or damage to the power supply. Often drawing excessive current can result in the magnetics involved in the power conversion process to become magnetically saturated, causing them to lose inductance, thereby rendering them ineffective. Since most uninterruptible power supplies involve the use of transformers of some sort, which electrically isolate the input from the output, when the magnetics become saturated the output voltage falls to zero almost immediately.
Additional detrimental effects are further possible. When the core of the transformer has saturated, all of the magnetizing inductance is eliminated from the primary winding. As the output voltage decreases, due to the current draw of the motor, the control circuitry in the uninterruptible supply will typically respond by increasing the duty cycle or on time of the magnetics. However as the duty cycle increases and the inductance in the primary winding falls to zero, the current through the primary winding which is approximated by the equation I.sub.L =(V.sub.L.times.T.sub.ON)/L becomes very large and potentially damaging to the other circuit elements, like switching semiconductors.
Correspondingly, there is a need for a DC-to-AC power converter circuit which avoids the use of magnetic transformers, and is capable of handling large currents associated with starting a motor.